The present invention relates to foamable thermoplastics. More particularly, the invention concerns polyetherimides containing effective amounts of a chemical blowing agent.
Polyetherimides are unique polymers which exhibit superior physical and chemical properties, including high heat resistance, exceptional strength and excellent processability. These polymers can be used as wire coatings and are particularly suited for injection molding applications. Because of their excellent properties, these polymers are often used in high performance applications, where they are exposed to a variety of conditions and high mechanical loads. Although polyetherimides are excellently suited to such applications, there is a continuing need for high performance and yet light weight polymers, particularly those that can be formed into useful articles by the relatively convenient and inexpensive technique of injection molding.
As will be more fully described herein, polyetherimides may be made by a variety of synthetic procedures. Presently, the preferred procedures for the commercial production of these polymers involves a step known as "melt polymerization." In the melt polymerization process, a mixture of an aromatic bis(ether anhydride) and an organic diamine (or a prepolymer thereof) is introduced into a heated extruder, wherein the reactants are melted. The reaction of these components occurs in the molten state, and the molten product is extruded through a die, after which it is cooled and chopped into pellets that can be conveniently utilized in injection molding applications. The processing temperatures associated with these production techniques can be quite high. For example, temperatures on the order of 400.degree. C. and higher are employed in the extruder. Moreover, because of the high glass transition temperatures of these resins, injection molding must generally be conducted at similarly high temperatures.
A wide variety of conventional polymers are foamed by the use of chemical blowing agents. Such polymers include polystyrene, polyethylene, polypropylene, polycarbonate, ABS, modified phenylene oxide, polyurethanes, and polyesters. Numerous chemical blowing agents have been developed and are employed for producing various foamed polymers. Such blowing agents include inert gases hydrocarbons such as pentane, or fluorocarbons, halocarbons, and unsymetrical t-alkylazoalkanes, to name a few. For a review of the factors involved with the foaming of polymers using chemical blowing agents. See Modern Plastics Encyclopedia, 1982-1983, pp. 272-289.
Because of the elevated temperatures involved in the production and processing of polyetherimides, imparting foamability to the polymer by incorporation of chemical blowing agents has proved to be difficult. Typical high temperature chemical blowing agents such as 5-phenyltetrazole, decompose to yield aromatic amines which can deleteriously affect the polymer. Chemical blowing agents typically decompose at a temperature far below the glass transition temperature of the polyetherimide.
Important considerations for the selection of a chemical blowing agent include the compatiblity of the blowing agent with the polymer system, the temperature at which decomposition takes place, and the effect of the blowing agent on various physical properties of the polymer including rigidity, density, strength and flexibility. In general, foamed polymers using chemical blowing agents have heretofore been limited to relatively low temperature applications. For example, polymers that are manufactured and processed at temperatures lower than about 175.degree. C. and usually below about 150.degree. C. The chemical blowing agents used to foam such polymers have generally been found unsatisfactory for the foaming of polyetherimides, because of one or more of the considerations recited above. In particular, such chemical blowing agents have met with limited success for polyetherimides because of the high processing temperatures employed and the adverse effects of products resulting from the decomposition of the blowing agent.
Accordingly, there is a continuing need for foamed polyetherimides whose useful properties meet certain standards not met by those polymers presently available.